The present invention relates to an axially symmetrical magnetron sputtering source and to a method of using the source for coating optical storage disks such as DVDs or photo DCs, preferably CD-R or phase change disks.
Magnetron sputtering sources are described in general in DE-PS 40 18 914; DE-OS 24 31 832; EP-A 0 330 445; EP-A 0 311 697; and U.S. Pat. No. 5,164,063.
EP-A 94105388.6 describes a magnetron sputtering source that uses a focusing target body arrangement which greatly improves the ratio between the amount of material that has been sputtered off and that is actually deposited on the workpiece in the form of a film. This approach significantly enhanced economic production, principally by the sputtering surface of the target body essentially defining the process space. Thereby, aside from the actual workpiece, only very small surfaces that can adversely affect the material transfer factor from the target to the workpiece are exposed to the material vapor.
The foregoing advantage is principally achieved by giving the target a special, conical or concave mirror shape that is focused on the workpiece surface. The disadvantage of this known arrangement is, however, that not all targets of that special shape can be manufactured for all sputtering materials with equal ease and that the special magnetic fields required for achieving magnetron discharge cannot be realized with the necessary ease in order to achieve special process characteristics. For example, the focusing shape of the target would have to be elaborate for creating precious metal targets.
The special shape can also result in problems if precious metal targets require special alloys or grain structures for process-related reasons. Another point, as mentioned above, is the optimization of the magnetic field configuration, whether it is due to the sputtering characteristic or to process-related energy conditions which are necessary due to the sensitivity of certain substrates.
It is also known to provide several toroidal sputtering zones, for example two zones, in order to achieve highly uniform film thickness distribution on a planar substrate. For example, EP 0 095 211 shows such an arrangement in which two concentric plasma rings are generated on the surface of a planar target by magnetron magnetic fields. The usual optimization is for good target utilization and film thickness distribution and is achieved with wide adjacent erosion pits and a relatively large target-to-substrate distance. In conjunction with the proposed devices such as reflectors, the large target distance is necessary to prevent the emitted charge carrier such as electrons from reaching and damaging, or heating, the target. Due to the large target-to-substrate distance and the installed deflection devices, the material yield or material transfer factor is unfavorable.
Certain applications for optical storage disks require the economic deposition of high-quality precious metal films without damaging the storage disks. In particular, for rewritable compact disks called CD-R, thin gold films with very specific properties are required. Similar films are also used for digital video disks known as DVD, or for photo CDs. Another field of application is the coating of memory disks that function according to the phase change principle. The precious metal target should be easy to manufacture, easy to recycle, and be able to fulfill certain metallurgical requirements. Due to the costly materials, a high transfer factor or yield of the sputtered material is necessary. Not only must the coating achieve very high reflectivity but care must be taken that the deposition process does not, through radiation or particle bombardment, damage the sensitive organic workpiece coatings on which the precious metal film, preferably gold, is to be deposited. The principles and the requirements of such coatings are described, for example, in Magnetic and Optical Media Seminar (Nov. 3 to 4, 1994, Atlanta), and in the corresponding seminar paper "The CDR: Yesterday, Today and Tomorrow" by Tad Ishiguro.
The disadvantage of such a sputtering source is that it cannot completely fulfill the aforementioned requirements. In particular, a magnetron discharge cannot be configured by magnetic fields such that the damage to the organic dye coatings of the workpiece can be minimized without having to reduce the high transfer rate when a simple, economic target arrangement is used.